An actuator with an electric motor and a method of controlling the electric motor to maintain a current position

ABSTRACT

For controlling an electric motor to maintain a current motor position, a motor controller determines the current motor position of the electric motor and selects (S 41 ) from a set of stable positions, defined by cogging torque of the electric motor, a selected stable position closest to the current motor position. The controller controls the motor torque of the electric motor to maintain the motor position (S 4 ) within a defined range around the selected stable position.

FIELD OF THE INVENTION

The present invention relates to an actuator with an electric motor anda method of controlling the electric motor to maintain a currentposition. Specifically, the present invention relates to an actuatorcomprising an electric motor having cogging torque, and a controllerconfigured to control operation of the electric motor and determine amotor position of the electric motor, and to a method of controlling theelectric motor to drive an actuated part of a Heating, Ventilating, andAir Conditioning (HVAC) system to a target position and maintain acurrent motor position.

BACKGROUND OF THE INVENTION

Actuators, which comprise an electric motor in connection with acontroller for controlling operation of the electric motor, are used inmany areas of application where parts need to be actuated by theelectric motor in a controlled fashion. Particularly, in applicationswhere mechanical parts need to be actuated into defined positions and/ororientations, actuators are equipped with controllers that areconfigured to determine the current motor position of the electric motorand to control operation of the electric motor, such as to move theelectric motor or the actuated part, respectively, to a set targetposition and/or orientation. Examples of application include actuatingand positioning dampers or shutters of fluid ducts or ports, regulatingmembers of valves, e.g. balls in ball valves or discs in disc valves, orthe like. In scenarios and situations where there is dynamicallychanging external influences and forces affecting the actuated parts,e.g. wind on the shutter of an air intake port, varying pressure in afluid transportation system, etc., the current position of the electricmotor or the actuated part, respectively, must be monitored continuouslyand readjusted, if necessary, in order to maintain a set targetposition. Maintaining the target position of the electric motor or theactuated part, respectively, in the presence of external forces andinfluences on the actuated parts may require significant amounts ofelectric energy. For the actual operation of the electric motor aninherent cogging torque is a further undesirable influential factor.Cogging torque of electrical motors is produced as a result of theinteraction between permanent magnets of the rotor and the stator slots.The cogging torque is especially prominent at lower speeds and can beobserved as stuttering or jerky movement.

EP 2491640 describes a brushless direct current motor with coggingtorque.

In the field of couplings for transmitting rotation, clutches andbrakes, DE 19843123 describes an electric brake for motor vehicles. Theelectric brake of DE 19843123 has a blocking brake function produced bypronounced stator and rotor poles and applying defined blocking currentto bring poles into latching position.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an actuator with anelectric motor and a method of controlling the electric motor to drivean actuated part of an HVAC system to a target position and maintain acurrent position. In particular, it is an object of the presentinvention to provide an actuator with an electric motor having coggingtorque, and a method of controlling the electric motor to drive anactuated part of an HVAC system to a target position and maintain acurrent motor position, whereby at least some electric energy requiredfor maintaining a target position can be reduced.

According to the present invention, the above-mentioned objects areparticularly achieved in that in an actuator, which comprises anelectric motor having cogging torque and a controller configured tocontrol operation of the electric motor and determine a motor positionof the electric motor, the controller comprises a circuit configured tocontrol the electric motor to drive an actuated part of an HVAC systemto a target position and maintain a current motor position bydetermining from a set of stable positions defined by the cogging torquea selected stable position closest to the current motor position, andcontrolling a motor torque of the electric motor to maintain the motorposition within a defined range around the selected stable position. Forexample, the electric motor is a brushless direct current motor.

In an embodiment, the circuit is configured to reduce the motor torqueas long as the current motor position is within the defined range aroundthe selected stable position.

In a further embodiment the circuit is configured to keep the motortorque at zero when the motor torque has been reduced to zero and thecurrent motor position is within the defined range around the selectedstable position.

In an embodiment, the circuit is configured to increase the motor torqueto return the motor position within the defined range around theselected stable position when the current motor position has movedoutside the defined range around the selected stable position.

In a further embodiment, the circuit is configured to record a value ofthe motor torque when the current motor position has reached a boundaryof the defined range around the selected stable position, to increasethe motor torque to return the motor position within the defined rangearound the selected stable position, and to reduce the motor torque aslong as the current motor position is within the defined range aroundthe selected stable position, using the recorded value of the motortorque for determining a limit of reducing the motor torque.

In an embodiment, the circuit is configured to determine the set ofstable positions by controlling the electric motor to move inincremental steps, determining the motor torque to maintain the motorposition at the incremental steps, and determining the set of stablepositions from the incremental steps requiring the smallest motor torqueto maintain the motor position.

In a further embodiment, the actuator further comprises an electricalenergy store configured to drive the electric motor to a defined safetyposition in case of a power failure.

In addition to the actuator, the present invention also relates to adamper for an HVAC system comprising a damper blade and the actuatorcoupled to the damper blade for moving the damper blade.

In addition to the actuator and the HVAC damper, the present inventionalso relates to a method of controlling an electric motor to drive anactuated part of an HVAC system to a target position and maintain acurrent motor position. The method comprises a controller determiningthe current motor position of the electric motor, the controllerdetermining from a set of stable positions, defined by cogging torque ofthe electric motor, a selected stable position closest to the currentmotor position, and the controller controlling a motor torque of theelectric motor to maintain the motor position within a defined rangearound the selected stable position.

In an embodiment, the controller reduces the motor torque as long as thecurrent motor position is within the defined range around the selectedstable position.

In a further embodiment, the controller keeps the motor torque at zerowhen and if the motor torque has been reduced to zero and the currentmotor position is within the defined range around the selected stableposition.

In an embodiment, the controller increases the motor torque to returnthe motor position within the defined range around the selected stableposition when and if the current motor position has moved outside thedefined range around the selected stable position.

In a further embodiment, the controller records a value of the motortorque when the current motor position has reached a boundary of thedefined range around the selected stable position, the controllerincreases the motor torque to return the motor position within thedefined range around the selected stable position, and the controllerreduces the motor torque as long as the current motor position is withinthe defined range around the selected stable position, using therecorded value of the motor torque for determining a limit of reducingthe motor torque.

In an embodiment, the controller determines the set of stable positionsby controlling the electric motor to move in incremental steps,determining the motor torque to maintain the motor position at theincremental steps, and determining the set of stable positions from theincremental steps requiring the smallest motor torque to maintain themotor position.

In addition to the actuator, the HVAC damper, and the method ofcontrolling the electric motor, the present invention also relates to acomputer program product, particularly, to a computer program productcomprising a non-transient computer readable medium. The computerprogram product or the non-transient computer readable medium,respectively, has stored thereon computer program code configured tocontrol a processor of an actuator such that the processor controls anelectric motor of the actuator to drive an actuated part of an HVACsystem to a target position and maintain a current motor position, bydetermining the current motor position of the electric motor,determining from a set of stable positions, defined by cogging torque ofthe electric motor, a selected stable position closest to the currentmotor position, and controlling a motor torque of the electric motor tomaintain the motor position within a defined range around the selectedstable position.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in more detail, by way ofexample, with reference to the drawings in which:

FIG. 1: shows a block diagram illustrating schematically an actuatorcomprising an electric motor, a motor controller with a control circuit,and an energy store.

FIG. 2: shows a block diagram illustrating schematically an actuatorcomprising an electric motor, a motor controller with a control circuit,and an energy store, the control circuit comprising a holding torquecontroller.

FIG. 3: shows a block diagram illustrating schematically an actuatorcomprising an electric motor, a motor controller with a control circuit,and an energy store, which actuator is mechanically coupled to a damperblade of a damper.

FIG. 4: shows graphs illustrating courses of magnetic energy and coggingtorque of an electric motor depending on motor position or angle,respectively.

FIG. 5: shows a flow diagram illustrating an exemplary sequence of stepsfor controlling and holding the motor position of an electric motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 to 3, reference numeral 1 refers to an actuator comprising anelectric motor 10, specifically an electric motor 10 which has coggingtorque, e.g. a brushless direct current motor, and a motor controller 11comprising a control circuit 12. The actuator 1 is an HVAC actuatorconfigured to drive an actuated part of an HVAC system to a targetposition, i.e. a set actuation position or actuated position within arange of actuatable positions, e.g. in a range from a fully closed to afully open position, or from a defined minimum position to a definedmaximum position. As illustrated in FIGS. 1 to 3, the actuator 1 furthercomprises an energy store 14, e.g. a battery or a capacitor, e.g. asupercapacitor (SC) such as a Lithium-ion capacitor (LIC). The energystore 14 is configured to power the electric motor 10, specifically inan emergency situation with power failure, such as to drive the electricmotor 10 and an actuated part actuated by the electric motor 10 to adefined safety position. For example, in an emergency situation, theenergy store 14 powers the electric motor 10 to drive a damper to aclosed or fully open position, depending on the respective applicationand scenario. The control circuit 12 comprises a programmable processor,an application specific integrated circuit (ASIC), or another electroniccircuit configured to control the electric motor 10. In theconfiguration involving a programmable processor, the actuator 1 furthercomprises or is connectable with a computer program product, whichcomprises a non-transient computer readable medium, having storedthereon programmed software modules with computer program codeconfigured to control the processor, such that the processor controlsthe electric motor 10 to maintain a current motor position as describedbelow in more detail.

As illustrated in FIG. 2, the control circuit 12 comprises variousfunctional modules which are implemented as electronic sub-circuits orprogrammed software modules controlling a processor, respectively. Thefunctional modules include a position controller 121, a speed controller122, a holding torque controller 123, a limiter 124, a currentcontroller 125, and a position feedback module 126. The positioncontroller 121 is configured to control the electric motor 10 or motorcurrent, respectively, to move to a set target position, defined by anumber of motor rotations, or an angle or position of an actuated part.The speed controller 122 is configured to control the speed of the motoraccording to a set motor speed. The holding torque controller 123 isconfigured to control the motor current or torque, respectively, such asto maintain a current motor position, as explained below in more detailwith reference to FIG. 5. The limiter 124 is configured to control themotor current within set limits of power, current, torque, and/or motortemperature. The current controller 125 is configured to control themotor current depending on control signals from the position controller121, the speed controller 122, the holding torque controller 123, and/orthe limiter 124. The position feedback module 126 is configured todetermine and provide the current position of the electric motor 10and/or its actuated part, respectively.

In FIG. 3, reference numeral 2 refers to a damper, specifically a damperfor a Heating, Ventilating, and Air Conditioning (HVAC) system. Asillustrated in FIG. 3, the damper comprises an actuated part 21,specifically a damper blade 21 for adjusting the orifice of the damper 2and thereby the flow of fluid, e.g. air, through the damper 2. Asfurther illustrated in FIG. 3, the actuated part 21, i.e. the damperblade 21, is mechanically coupled to the actuator 1 by way of amechanical coupling 22, e.g. a drive shaft, for actuation by theactuator 1 or its electric motor 10, respectively. The actuator 10 orits motor 10, respectively, drives or moves the actuator part 21, i.e.the damper blade 21.

FIG. 4 shows in the upper graph the course of the magnetic energy E ofthe electric motor 10, as a function of or depending on the motor angleΦ (or the motor position, respectively). The lower graph of FIG. 4 showsthe course of the cogging torque C of the electric motor 10, as afunction of or depending on the motor angle Φ (or the motor position,respectively). As indicated in FIG. 4, the position or angle Φ of theelectric motor 10 has stable positions P1, P2 and (stable) ranges R1, R2around these stable positions P1, P2 where positive (+) cogging torque Cand negative (−) cogging torque C draws the electric motor 10 towardsthe stable positions P1, P2; whereas in (instable) ranges aroundinstable positions P3 positive (+) cogging torque C and negative (−)cogging torque C pulls the electric motor 10 away from the instablepositions P3.

As illustrated in FIG. 5, in preparatory step S0, the motor controller11 or its circuit 12, respectively determines the stable positions P1,P2 of the electric motor 10 (position or angle ED). Furthermore, themotor controller 11 or its circuit 12, respectively determines definedranges R1, R2 around the stable positions P1, P2, e.g. as portion of thedistance or difference d between two consecutive stable positions P1,P2, d=P2−P1, e.g. a range R of R=[P−25%·d; P+25%·4] around a stableposition P. For example, in a motor configuration where stable positionsP1, P2 occur every 20°, i.e. d=20°, the range R around a stable positionP is defined by R=[(P−5°)<Φ; Φ>(P+5°)], e.g. around P1: R1=[−5°<Φ;Φ<5°], or around P2: R2=[15°<Φ; Φ<25°]. Specifically, the stablepositions P1, P2 (and ranges R1, R2) are stored in a data store of themotor controller 11 or its circuit 12, respectively. Depending on theembodiment and/or configuration, the stable positions P1, P2 (and rangesR1, R2) are determined by performing a calculation, based on a knownconfiguration of the magnetic poles of the stator and the rotor of theelectric motor 10, specifically based on the number of magnetic poles onthe stator (e.g. an internal stator with nine magnetic poles) and thenumber of magnetic poles on the rotor (e.g. an external rotor with sixmagnetic poles) of the electric motor 10, or by performing a measurementrun of the electric motor 10. For example, the calculation is performed“off-line” for the particular type and (magnetic) configuration of theelectric motor 10 and stored in the motor controller 11 at manufacturingtime or a later point in time. In case of the measurement run, thecircuit 12 of the motor controller 11 controls the electric motor 10 tomove in incremental steps, e.g. a rotation of one degree or of a partialdegree, and determines the motor torque required to maintain the motorposition at the incremental steps. Subsequently, the set of stablepositions are determined as those incremental steps which require thesmallest motor torque to maintain the motor position.

In step S1, the motor controller 11 or its circuit 12, respectively,receives or sets a target position for the electric motor 10 or a targetvalue that relates to a target position for the electric motor 10.Depending on the application and/or installation, the target position ortarget value is defined and set by a building control system or a userterminal communicatively connected to the electric motor 10.

In step S2, the motor controller 11 or its circuit 12, respectively,controls the electric motor 10 to move to the set target position, e.g.to perform a certain number of rotations corresponding to a set rotaryposition (angle) or for driving an actuated part to a set (actuated)position. As illustrated schematically in FIG. 2, the motor controller11 uses the position controller 121, the speed controller 122, thelimiter 124, and the current controller 125 to control the electricmotor 10 to reach the target position.

In step S3, the motor controller 11 or its circuit 12, respectively,determines whether the electric motor 10 or the actuated part,respectively, has reached the target position. Specifically, theposition feedback module 126 determines and indicates the currentposition of the electric motor and/or the actuated part driven by theelectric motor 10. If the target position has not been reached yet,control of the electric motor 10 is continued in step S2; otherwise, ifthe target position has been reached, a process of maintaining thecurrent motor position is activated. For example, for that purpose, theposition feedback module 126 or the circuit 12 or motor controller 11,respectively, activates the holding control torque controller 123.

In step S4, the holding control torque controller 123 or the circuit 12or motor controller 11, respectively, controls the electric motor 10 tomaintain its position at the current position or target position,respectively, as described below in more detail with reference tosub-steps S41, S42, S43, S44, and S45. One skilled in the art willunderstand that external forces and influences, such as a wind gust on adamper blade 21 of an external air damper 2 or a pressure change insidea fluid duct, will have an impact on the motor position and will have tobe compensated by adapting the motor torque or motor current,respectively, to hold against the external force or influence, such asto maintain a target position.

In step S41, the holding control torque controller 123 or the circuit 12or motor controller 11, respectively, determines the stable positionclosest to the target position or current position of the electric motor10.

In step S42, the holding control torque controller 123 or the circuit 12or motor controller 11, respectively, checks whether the currentposition of the electric motor 10 is within the range R1, R2 around thestable position P1, P2 determined in step S41. If the current positionis within the respective range R1, R2, the process proceeds in step S43;otherwise, if the current position is outside the respective range R1,R2, processing proceeds in step S45 by increasing the motor torque ormotor current, respectively.

In step S43, the holding control torque controller 123 or the circuit 12or motor controller 11, respectively, checks whether the current motortorque or the motor current, respectively, is at zero, i.e. whether theelectric motor 10 maintains its current position without requiring anymotor current and, thus, not producing motor torque. If the currentmotor torque or the motor current, respectively, is at zero, the torqueis maintained at zero and the processing continues in step S42.Otherwise, if the current motor torque or the motor current,respectively, is not at zero, processing continues in step S44 byreducing the motor torque or motor current, respectively.

In step S44, the holding control torque controller 123 or the circuit 12or motor controller 11, respectively, reduces the motor torque or themotor current, respectively, e.g. by a predetermined amount or portion.

In an embodiment, the holding control torque controller 123 or thecircuit 12 or motor controller 11, respectively, determines theduration, e.g. in terms of time or number of cycles, during which themotor torque or the motor current, respectively, is not at zero. If this“non-zero torque duration” is longer than a defined threshold, e.g. oneminute, five minutes or an hour, processing continues in step S0 bydetermining an alternative stable position. For example, the alternativestable position is the stable position that is located preceding(before) or succeeding (following) the current stable position(previously selected in step S0). This approach makes it possible tofind more advantageous stable positions which require less motor torqueor motor current, respectively, through “trial and error”.

In step S45, the holding control torque controller 123 or the circuit 12or motor controller 11, respectively, increases the motor torque ormotor current, respectively, e.g. by a predetermined amount or portion,and proceeds in step S42.

In an embodiment, in step S45, the holding control torque controller 123or the circuit 12 or motor controller 11, respectively, furtherdetermines and stores the motor torque present at the point where themotor position has reached an upper or lower boundary b1L, MU, b2L, b2Uof the defined range R1, R2 around the selected stable position P1, P2.Subsequently, when the motor torque or motor current, respectively, isreduced in step S44, it is not reduced beyond said recorded motortorque, which is thus used as a limit for reducing the motor torque.

It should be noted that, in the description, the computer program codehas been associated with specific functional modules and the sequence ofthe steps has been presented in a specific order, one skilled in the artwill understand, however, that the computer program code may bestructured differently and that the order of at least some of the stepscould be altered, without deviating from the scope of the invention.

1. An actuator comprising: an electric motor having cogging torque, anda controller configured to control operation of the electric motor anddetermine a motor position of the electric motor, wherein the controllercomprises a circuit configured to control the electric motor to drive anactuated part of a Heating, Ventilating, and Air Conditioning system toa target position and maintain a current motor position by determiningfrom a set of stable positions defined by the cogging torque a selectedstable position closest to the current motor position, and controlling amotor torque of the electric motor to maintain the motor position withina defined range around the selected stable position.
 2. The actuator ofclaim 1, wherein the circuit is configured to reduce the motor torque aslong as the current motor position is within the defined range aroundthe selected stable position.
 3. The actuator of claim 2, wherein thecircuit is configured to keep the motor torque at zero when the motortorque has been reduced to zero and the current motor position is withinthe defined range around the selected stable position.
 4. The actuatorof claim 1, wherein the circuit is configured to increase the motortorque to return the motor position within the defined range around theselected stable position when the current motor position has been movedoutside the defined range around the selected stable position.
 5. Theactuator of claim 1, wherein the circuit is configured to record a valueof the motor torque when the current motor position has reached aboundary of the defined range around the selected stable position, toincrease the motor torque to return the motor position within thedefined range around the selected stable position, and to reduce themotor torque as long as the current motor position is within the definedrange around the selected stable position, using the recorded value ofthe motor torque for determining a limit of reducing the motor torque.6. The actuator of claim 1, wherein the circuit is configured todetermine the set of stable positions by controlling the electric motorto move in incremental steps, determining the motor torque to maintainthe motor position at the incremental steps, and determining the set ofstable positions from the incremental steps requiring the smallest motortorque to maintain the motor position.
 7. The actuator of claim 1,wherein the actuator further comprises an electrical energy storeconfigured to drive the electric motor to a defined safety position incase of a power failure.
 8. The actuator of claim 1, wherein theelectric motor is a brushless direct current motor.
 9. A damper for aHeating, Ventilating, and Air Conditioning system comprising a damperblade and an actuator according to claim 1, coupled to the damper bladefor moving the damper blade.
 10. A method of controlling an electricmotor to drive an actuated part of a Heating, Ventilating, and AirConditioning system to a target position and maintain a current motorposition, the method comprising: determining, by a controller, thecurrent motor position of the electric motor, determining, by thecontroller, from a set of stable positions, defined by cogging torque ofthe electric motor, a selected stable position closest to the currentmotor position, and controlling, by the controller, a motor torque ofthe electric motor to maintain the motor position within a defined rangearound the selected stable position.
 11. The method of claim 10, furthercomprising reducing, by the controller, the motor torque as long as thecurrent motor position is within the defined range around the selectedstable position.
 12. The method of claim 11, further comprising keeping,by the controller, the motor torque at zero when the motor torque hasbeen reduced to zero and the current motor position is within thedefined range around the selected stable position.
 13. The method ofclaim 10, further comprising increasing, by the controller, the motortorque to return the motor position within the defined range around theselected stable position when the current motor position has been movedoutside the defined range around the selected stable position.
 14. Themethod of claim 10, further comprising recording, by the controller, avalue of the motor torque when the current motor position has reached aboundary of the defined range around the selected stable position,increasing, by the controller, the motor torque to return the motorposition within the defined range around the selected stable position,and reducing, by the controller, the motor torque as long as the currentmotor position is within the defined range around the selected stableposition, using the recorded value of the motor torque for determining alimit of reducing the motor torque.
 15. The method of claim 10, furthercomprising determining, by the controller, the set of stable positionsby controlling the electric motor to move in incremental steps,determining the motor torque to maintain the motor position at theincremental steps, and determining the set of stable positions from theincremental steps requiring the smallest motor torque to maintain themotor position.
 16. A non-transitory computer readable medium storingcomputer program code which, when executed by a processor of anactuator, causes the processor to control an electric motor of theactuator to drive an actuated part of a Heating, Ventilating, and AirConditioning system to a target position and maintain a current motorposition, by: determining the current motor position of the electricmotor, determining from a set of stable positions, defined by coggingtorque of the electric motor, a selected stable position closest to thecurrent motor position, and controlling a motor torque of the electricmotor to maintain the motor position within a defined range around theselected stable position.